The final top donor request (the rest didn’t request a topic):

“blog about this“

Since this is from a friend and not a stranger, I can safely say this: I hate you for making me read a scientific paper and review it during Blogathon. Hate. So much hate.

But you have my word, so I’ll do it. Onto the science!

A lot of people like to ask the question “What makes men gay?” It’s pretty clear it’s not a willy-nilly lifestyle choice, but scientists still aren’t really sure what the biology behind homosexuality actually is. Is it genetic? Hormonal?

Research on the latter is what a recent review article in the journal Frontiers in Neuroendocrinology summarized. You probably heard about the starting premise, since it received a lot of attention in the media. A study in 1996 found that gay men had a greater number of older brothers than heterosexual men. This is known as the “fraternal birth order” (FBO) effect, and has been replicated in many studies. It’s independent of potentially confounding variables like year of birth, age, socioeconomic status, and parental age. Non-biological siblings had no effect on sexual orientation.

The main hypothesis for why you see this pattern is known as the maternal immune hypothesis. Just like your body mounts an immune response against bacteria or ill-matched transplants, moms may develop an immune reaction against a male specific protein that’s present during development. Those proteins are normal for a male fetus, but a mother’s body still recognizes them as foreign. The immune response may then alter parts of the brain associated with male specific proteins like the anterior hypothalamus, which has also been linked to sexual orientation.

Recent research is finding more and more support for this hypothesis. One study showed that mothers of boys do develop an immune response to H-Y antigen, which is a protein expressed in the brain that is important in male fetal development. This immune response becomes stronger and stronger with each son a mother has.

This isn’t a totally crazy hypothesis. This exact thing happens in terms of blood type:

A medical model for a maternal immune response underlying the FBO effect is Hemolytic Disease of the Newborn (HDN). When a mother does not have the Rh factor in her blood (i.e., a mother is Rh negative), after gestating and giving birth to an Rh positive (Rh +) fetus, she may mount an immune response against the Rh factor. This immune response may affect subsequent Rh + fetuses, potentially attacking their red blood cells and causing anemia. The likelihood of an immune response becomes increasingly likely with each Rh + fetus a mother gestates.

There’s a problem though. H-Y antigen isn’t just produced in the brain – it’s also expressed in the gonads. Homosexual and heterosexual men don’t have any major difference in terms of gonads or fertility. Is there a way that the immune response would only effect H-Y proteins in the brain, but not in the gonads?

Possibly. Mice testes can develop without H-Y. And male gonads don’t reach maturity until puberty, so maybe a maternal immune response wouldn’t affect sperm too much.

The most compelling point is that there are three different forms of H-Y protein. It’s possible that the different forms are localized in different tissues, with only the one in the brain being targeted by the maternal immune response. However, there’s currently no information on where different forms of H-Y protein are localized.

Despite all of this evidence, this still doesn’t provide an actual mechanism. There’s a big gap between “increased immune response” to “homosexual behavior.” What are all of the steps in between? And is H-Y the only male specific protein that a maternal immune response targets? Probably not, but more research still needs to be done.

So do we definitively know what’s going on yet? Not quite. But feel free to slap homophobes with some science the next time you hear “lifestyle choice.”

And there goes all of the Blogathoon buffer time I had built up. Curses!

This is post 24 of 49 of Blogathon. Pledge a donation to the Secular Student Alliance here.